The health risks of mercury exposure are well documented, and the harms for still-developing fetuses are particularly concerning. Now, the latest study finds that kids who were exposed to more mercury in the womb were more likely to show problems with attention and hyperactivity and other symptoms of ADHD at age 8.

Complicating the matter, however, is that the same study found that children whose mothers ate more fish during pregnancy — fish is known to be a main source of mercury exposure for many people — had a significantly lower risk of ADHD symptoms than kids whose mothers ate less fish. The finding raises the possibility that the health benefits of eating fish, which is rich in the omega-3 fatty acids that are good for brain development, could outweigh the harms of low-level mercury exposure.

Previous studies attempting to define the risk of low-level exposure to mercury have been inconclusive: one large study showed that school-age children who were exposed to higher levels of mercury in utero — as measured by samples of their cord blood — showed more errors and lower scores on neurocognitive tests. But another study indicated that children exposed to higher levels of mercury were actually less likely to show problem behaviors such as hyperactivity or chronic inattention.

So Dr. Susan Korrick and her colleagues at Brigham and Women’s Hospital in Boston studied data on 788 infants born between 1993 and 1998 in the New Bedford, Mass., area. The researchers measured the mercury in the children’s mothers’ hair samples 10 days postpartum, and used a 59-item questionnaire to evaluate the children’s behavior at age 8.

The researchers found that higher mercury levels in the hair samples were associated with a greater risk of symptoms of ADHD, or attention deficit-hyperactivity disorder in children (the researchers did not confirm ADHD diagnosis, but only determined whether the children had symptoms that were typical of the disorder). Even after adjusting for potential factors that could influence the risk of ADHD symptoms, including the mother’s education or depression, or her diet while pregnant, the correlation with mercury remained.

Studies hint that mercury affects central nervous system development by disrupting levels of important neurochemicals such as dopamine.

“The message that prenatal mercury exposure is not healthy for the fetus is not changed by this study,” says Korrick. But because those studies have been inconsistent, Korrick says the results should “demonstrate fairly consistent findings related to ADHD behaviors.”

Based on the new data, Korrick says that the higher risk of ADHD symptoms starts to appear at mercury levels of 1 microgram per gram: rates of inattention and hyperactivity among kids whose mothers showed levels of exposure above that threshold ranged 40% to 70% higher than for those whose mothers had lower levels of mercury.

But that study doesn’t suggest that pregnant women should avoid eating fish altogether. The data also showed that children of women who ate more fish during pregnancy — more than two servings a week — were 60% less likely to have ADHD symptoms than kids whose mothers ate less. While that seems contradictory, Korrick points out that not all fish is high in mercury, and that eating more fish could have other benefits for brain and behavior development that are worth considering. The current study did not determine what kinds of fish women in the study ate, however.

This balance between the benefits and harms of fish consumption may also help explain the contradictory results of previous mercury studies. In those that show no increased risk of ADHD or other behavioral symptoms, the participants could have been heavy fish consumers overall, so the benefits of their diet for children’s neurocognitive development may have offset the harms from mercury. In contrast, in those studies showing a harmful effect of mercury at low levels, the negative effects of mercury may have overwhelmed any potential benefits of fish, if the participants had consumed primarily fish species that were high in mercury and perhaps low in omega-3s.

The U.S. Environmental Protection Agency urges pregnant women to limit their fish consumption to two 6-oz. servings a week in order to protect developing babies. Large, deep-water species such as tuna, swordfish and mackerel, which are among the more popular options on restaurant menus, also tend to be higher in mercury than smaller, oily fishes, such as salmon, herring and sardines. Learning which fish are nutritious but low in mercury — others include catfish, tilapia and whitefish — takes a bit of work, but organizations such as the Food and Drug Administration and the National Resources Defense Council have useful lists.

“It’s not necessarily contradictory,” says Korrick of the advice to eat fish, yet avoid mercury exposure. “It would be an unfortunate public health message if people stopped eating fish because of the concern over mercury. Understanding and appreciating that mercury avoidance is a prudent thing to do during pregnancy is important, but understanding and appreciating that fish consumption is important is worth remembering as well. Eating fish isn’t equivalent to getting exposed to mercury.”